In production of semiconductor devices typified by LSIs (large scale integrated circuits), photolithography is indispensable in order to subject an insulating film (e.g. a silicon oxide film or a silicon nitride film), a conductive film (e.g. an aluminum alloy film or a copper alloy film) both formed on a semiconductor substrate, or a to-be-processed material containing a semiconductor substrate, to patterning to be a desired shape.
In this photolithography, it has been conducted to coat an ultraviolet light-sensitive photoresist on a to-be-processed material to form a photoresist film and then apply an ultraviolet light to the photoresist film through a mask pattern to make the ultraviolet light-applied area of the film soluble (in the case of a positive-type photoresist) or insoluble (in the case of a negative-type photoresist). Then, the photoresist film is subjected to a development treatment to remove the soluble area partially with a solvent and form a resist pattern, after which the to-be-processed material is etched selectively using the resist pattern as a mask to conduct patterning.
As the photoresist, positive-type novolak-based photoresists have been used generally. Positive-type photoresists are superior in resolution to negative-type photoresists and therefore have been used mainly as the above photoresist. Meanwhile, as the light applied to the photoresist, ultraviolet lights such as g-line (wavelength: about 436 nm), i-line (wavelength: about 365 nm) and the like were used. However, in order to obtain a more precise resist pattern, a photolithography using a far-ultraviolet light consisting of a KrF (krypton fluoride) excimer laser beam (wavelength: about 248 nm) has been developed and come to be used.
With the higher integration of LSIs, a photolithography to form a finer pattern has become necessary. In this connection, the light to be applied to photoresists is being shifted to an ultraviolet light of shorter wavelength capable of giving a higher resolution. Consequently, in production of, in particular, a DRAM (dynamic random access memory) having an integration of 1 gigabit or more, where microfabrication technology of 0.13 μm or less is required, it has recently come to be considered to use a photolithography using an ArF (argon fluoride) excimer laser beam (wavelength: about 193 nm) which generates a far-ultraviolet light having a wavelength shorter than that of the above-mentioned KrF excimer laser beam [Donald C. Hoffer et al., Journal of Photopolymer Science and Technology, Vol. 9 (No. 3), pp. 387 to 397 (1996)].
When the above ArF excimer laser beam is applied to a conventional novolac-based photoresist mentioned above, however, it is difficult to from an excellent resist pattern because the novolac-based photoresist shows a high light absorption. Therefore, it is desired to develop a resist resin for the photolithography using an ArF excimer laser beam.
In development of a photoresist resin allowing the use of an ArF excimer laser beam, an improved cost performance of laser must be realized because the gas as a raw material of laser has a short life and the laser beam machine is costly. Therefore, a high sensitivity as well as a high resolution enabling microfabrication is required strongly for the photoresist resin.
As for the photoresist with a higher sensitivity, a chemically amplified resist using a photo acid generator (which is a sensitizer) is well known. It is described in, for example, JP-B-2-27660. The literature describes a resist comprising a combination of triphenylsulfonium hexafluoroarsenate and a poly(p-tert-butoxycarbonyloxy-α-methylstyrene). Such a chemically amplified resist is currently in wide use in resists for KrF excimer laser beam [for example, Hiroshi Ito and C. Grant Wilson, American Chemical Society Symposium Series, Vol. 242, pp. 11 to 23 (1984)]. The feature of chemically amplified resists lies in that the photo acid generator contained therein as a component generates a proton acid upon irradiation with a light and this proton acid gives rise to an acid-catalyzed reaction with the resist resin and the like, in a heat treatment after light application. Thus, there can be obtained a strikingly high sensitivity in the resists as compared with those of conventional resists with a photoreaction efficiency (a reaction per one photon) of less than 1. Most of the resists being developed currently are chemically amplified resists.
In a lithography using a far-ultraviolet light having a wavelength of about 220 nm or less, typified by an ArF excimer laser beam, however, the resist used therein for forming fine patterns needs to have novel properties which cannot be satisfied by the conventional materials, that is, high transparency to an exposing light having a wavelength of about 220 nm or less and the resistance to dry etching.
In the above-mentioned conventional photoresist resins for g-line (wavelength: about 438 nm), i-line (wavelength: about 365 nm) and KrF excimer laser beam (wavelength: about 248 nm), as the resin component, mainly a resin having an aromatic ring in the structural units, such as novolac resin, poly(p-vinylphenol) or the like is used, and can maintain the dry etching resistance owing to the dry etching resistance of the aromatic ring. Resins having an aromatic ring, however, show very high absorption to a light having a wavelength of about 220 nm or less. Therefore, the most part of the exposed light with the wavelength of about 220 nm or less is absorbed on the surface of the resist and the light does not reach a substrate. As a result of it, a fine resist pattern cannot be formed. Thus, the conventional resins are not applicable in the photolithography using a far-ultraviolet light of about 220 nm or less. There is accordingly a strong desire for a new resist resin, which has the high transparency to the far-ultraviolet light having a wavelength of about 220 nm or less without having the aromatic ring and has the resistance to the dry etching.
As polymer compounds having transparency to an ArF excimer laser beam (wavelength: about 193 nm) and further having dry etching resistance, there were proposed the alicyclic polymers, i.e. a copolymer having adamantyl methacrylate units [Takechi et al., Journal of Photopolymer Science and Technology, Vol. 5 (No. 3), pp. 439 to 446 (1992)] and a copolymer having isobornyl methacrylate units [R. D. Allen et al., Journal of Photopolymer Science and Technology, Vol. 8 (No. 4), pp. 623 to 636 (1995) and Vol. 9 (No. 3), pp. 465 to 474 (1996)] (these polymers are hereinafter referred to as former resin).
There was also proposed the resin having norbornene-maleic anhydride alternating copolymer units [F. M. Houlihan et al., Macromolecules, Vol. 30, pp. 6517 to 6524 (1997)] (the resin is hereinafter referred to as latter resin).
Each of the above-mentioned resist resins for a lithography using a far-ultraviolet light having a wavelength of about 220 nm or less, typified by an ArF excimer laser beam, however, has the following drawbacks.
First, the alicyclic group-containing (meth)acrylate derivative used in the former resin has no polar group (e.g. carboxy group or hydroxy group) having adhesivity to substrate. As for a homopolymer of an alicyclic group-containing monomer, the adhesivity with the substrate to-be-processed (e.g. silicon substrate) is not good because it has high hydrophobicity, and the uniform film formation at high reproducibility may be impossible. Further, the units of adamantane-containing residue, isobornyl-containing residue or menthyl-containing residue have no residue which shows the solubility difference before and after exposure. Therefore it is impossible to form a pattern by exposure. Thus, the former resin can be used as a resist resin only when made into a copolymer with a comonomer capable of exhibiting solubility difference, such as tert-butyl methacrylate, tetrahydro methacrylate or the like, or with a comonomer having adhesivity to substrate, such as methacrylic acid or the like. However, the content of the comonomer is required to be about 50 mole % and the comonomer units have very low resistance to dry etching. As a result, the dry etching resistance for the resin with alicyclic group decreases significantly by the copolymerization with the comonomer, and the practical applicability as a dry etching-resistant resin is low.
Next, the latter resin (the resin using a norbornene-maleic anhydride alternating copolymer) as well has low adhesivity to a substrate because the norbornane ring has no polar group. Therefore, the resin having the units of a norbornene-maleic anhydride alternating copolymer can be used by making it to copolymerization with acrylic acid or the like with a good adhesivity to the substrate. However, the practicality as the dry etching-resistant resin decreases.
Hence, there is a strong need for a novel positive-type chemically amplified resist which has high transparency to a far-ultraviolet light with a wavelength of about 220 nm or less, high etching resistance and high adhesivity to a substrate.